The temperature data measured with the microwave thermometer in the ground vacuum experiment suffer from a high-order nonlinearity, which cannot be properly calibrated with the traditional method. The origin of this high-order term was analyzed with the diode equation expressed in the form of Maclaurin expansion. A method combining the two-point linear correction with the cubic equation nonlinear correction was proposed to calibrate the deviation of the temperature data from the actual one. The method was verified through the cross-comparison of data from O-B and ATMS for the FY-3C microwave thermometer. The calibration precision reaches up to 4 K higher than that achieved using the U parameter correction method.